19.10 Java4660: sockets  (Page 3/8)

This module will be concerned with only the first two. However, to use any of them, you must configure your computer to support all of them.

Configuring your computer

Do the following to configure your computer to support these services.

In Windows 7, Vista, and possibly older versions of the Windows operating system, do the following:

• Open the Control Panel
• Select Programs and Features
• Select Turn Windows features on or off
• Scroll down to Simple TCPIP services and check the box
• Also check the box for Telnet Client
• Click OK and follow the directions

On some computers, this may take a long time to complete. (It took more than 15 minutes on an older laptop computer that I own.) .

Test using telnet

When the process is complete, open a command line window and enter the following:

telnet localhost 13

You should see something like the contents of Figure 1 if your computer is supporting simple TCP/IP services.

You can use localhost to do a query on the same machine or you can do a query on the name of some other properly configured computer on a local area network.

(You can also use the name of the same computer in place of localhost to do a query on the same computer.)

Opening ports on the firewall

In the case of accessing another computer on the network, in addition to the procedure described above , you may also need to open the required ports in the Windows firewall or perhaps a firewall that is beingmanaged by other software running on that computer.

(The ports of interest are shown in the above list .)

I won't attempt to explain how to open and close ports in the firewall. The Windows Help and Support feature explains how to open and close ports in the Windows firewall. If the firewall is being managed by another program, the documentation for that programshould explain how to open and close ports.

Echo program

This program implements a client that performs a simple echo test by sending a line of text to the echo port (port 7) on a server (or on the same computer) .

Overview

The program begins by instantiating a String object containing the name of an echo server that is being used to test the program (localhost as the program is shown in Listing 1 ) . This is followed by the declaration and initialization of an int variable containing the standard echo port number (7) .

Than the program gets a socket connection to port 7 on the server as shown in Listing 2 ..

Following this, the program gets input and output streams from the socket and wraps them in the Reader and Writer classes as shown in Listing 3 .

Once the connection is made and the input and output streams are ready to use, the program sends a line of text to the echo port on the specified server as shown in Listing 4 . This causes the server to send the same line of text back to the client.

The program reads the line of text that is received and displays it in the command-line window.

Finally, the program closes the socket and terminates.

The program output